The present invention relates generally to microphones that convert acoustic signals to electrical signals and, more particularly, to a microphone that may be subjected to both substantial vibration and sound pressure. Such an environment may exist, for example, in an automotive vehicle active noise cancellation system.
In such a system, a microphone may be attached to an exhaust pipe, or other component, of an automobile. Signals from the microphone may then be interconnected to a synthesizer, or other computer, to analyze the acoustic signals received by the microphone. Thereafter, the synthesizer responsively provides cancelling, counter-phase acoustic signals to, in effect, "cancel out" the noise of the automobile engine.
Such noise cancellation systems are often used to "muffle" an engine's noise, while minimizing the back pressure on the engine that may be induced with other types of noise mufflers. As a result, active noise cancellation systems result in greater power and fuel economy than if more conventional, passive mufflers are used.
The microphone in the cancellation system may be used to measure what acoustic signals are being generated by the car engine at a particular time (with the engine at a particular power level). This enables the synthesizer to cause the correct acoustic signal to be produced.
The microphone may also be used to sense how well the cancellation system has achieved its objective of providing counter-phase, nullifying noise. Accordingly, the microphone may be used as a part of a feedback system to tune the phase of the cancellation signal produced by the synthesizer.
The environment in which such a noise cancellation system operates is particularly harsh for a microphone. The microphone is subjected to substantial vibration as the vehicle travels. A transducer within a microphone (which converts acoustic signals into electrical signals) may be affected not only by acoustic signals, but the vibration as well. As a result, the microphone may produce an electrical signal which, rather than being truly representative of the sound received by the microphone, instead represents a combination of both the sound and the vibration that the microphone happens to be experiencing.
Such a microphone in an automotive active nose cancellation system may be subjected to intense heat from the vehicle's exhaust system. Such microphones may be required to operate at elevated temperatures of up to 3500.degree. F. or more.
Moreover, such microphones must operate with the high sound levels of automotive engines. The sound pressures generated by an engine may be as high as 175 db SPL or more.
Nonetheless, such microphones should be relatively small in size and inexpensive to construct, so that they may be more readily attached to mass-produced automotive vehicles. The microphones should also operate at frequencies below 500 hertz, since such lower frequency sounds are generally the most difficult for conventional, passive muffler systems to control.
Microphones are also used in other types of harsh environments, such as, for example, as part of a knock-sensor assembly on an automobile engine. Such microphones are positioned in the engine compartment to sense the operation of the engine pistons. Of course, such microphones are subjected to extensive vibration, high sound pressure, and elevated temperatures, and must still be small and inexpensive.
Many conventional microphones are poorly suited for use in an environment of high vibration, sound pressure, and temperature. In response to being vibrated, some microphones may add spurious signals to the electrical signals representing the sound. Others cannot withstand intense sound pressure or high temperatures without malfunctioning. Others have substantially reduced sensitivity to both noise as well as vibration, reducing the effectiveness of the microphones.
One technique of reducing the vibration sensitivity of a microphone is to keep the mass of the transducer small. However, in a severe environment, this technique also reduces the physical strength of the transducer. Thus, low transducer-mass microphones may be inappropriate for use in an environment of intense vibrations, high sound pressures, and elevated temperatures.